Electric calcining of carbonaceous material such as anthracite, pitch coke, metallurgical coke and petrol coke is carried out by conducting electric current through a bed of the carbonaceous materials which are to be calcined. Electric current is supplied to the calcining furnace through one carbon electrode arranged in the top of the furnace and one carbon electrode arranged in the bottom of the furnace. The calcining furnace is in the form of a vertical shaft where the material which shall be calcined, is supplied at the top of the furnace. The flow of material through the furnace is controlled by the rate of discharge of calcined material at the bottom of the furnace.
During the calcining process the carbonaceous material is heated and upon heating any moisture will be removed from the carbonaceous material. Upon further heating volatile matters is released from the carbonaceous material. Depending on the final temperature to which the carbonaceous material is being heated, a part of the carbonaceous material will be transformed to graphite.
Electric calcining of carbonaceous material is characterized in that the temperature between the electrodes in the center of the furnace is very high, above 2500.degree. C., while the temperature in the periphery of the furnace is substantially lower, between 800 and 1200.degree. C. The finished calcined product removed from the lower end of the furnace has for this reason been subjected to an uneven heat treatment during the calcining process. The very high temperatures in the center of the furnace results in that ash components in the carbonaceous materials are volatalized and transferred radially outwards where they condense on colder material or on the inner furnace wall lining. In order to be able to run the calcining process in a continuous and stable way, the mass balance in the furnace has to be maintained. This means the ash components in the carbonaceous material supplied at the top of the furnace, must be withdrawn together with the calcined material at the bottom of the furnace.
In conventional electric calcining furnaces withdrawal of calcined material at the bottom of the furnace is done over the whole furnace cross-section, whereby a mixture of material which has been heated within a temperature range between 800.degree. C. to above 2500.degree. C., is obtained. This gives an inhomogeneous calcined material with respect to ash content, content of graphite and electrical conductivity.